Automotive vehicles, such as trucks, have a supporting ladder frame on which a passenger compartment and a cargo supporting bed are attached. The frame has a suspension system and supporting wheels depending therefrom. The ladder frame typically employs two frame side rails that are interconnected by transverse beam members. These components, side rails and transverse beams are welded to form the ladder frame. As a general rule the frame side rail is constructed from many components welded together. In many instances both rectangular tube and channel structures are used in a welded frame assembly with the end components being tubular and the center components being channel structures.
The multiple components of the frame side rails are welded into a single structure. Each of the components has a unique shape. For example, the fore and aft end pieces have one cross-sectional area and the center piece or pieces have a second cross-sectional area. The fore and aft pieces are joined with the center piece through a transitional piece which has cross-sectional areas to match the adjoining pieces. The wall thickness of various pieces, however, is substantially constant.
The wall thickness is determined by the component needing the most mass to provide the required support. Generally this is the fore and aft pieces which are attached to the suspension components. The center pieces have more thickness than required and therefore more mass and weight. The center pieces generally need a larger height to width ratio cross-section to accommodate the required torsional rigidity and bending strength of the frame during vehicle operation.
Many of the prior art frame side rails are constructed by roll forming the various components of the side rail. A bending process is employed to form a side rail member having a rectangular cross section. Following the bending process, the abutting edges are welded to complete the rectangular tube. This process has been replaced in many designs with a hydroforming process. However, the center sections of the side rails remain as "C" channel sections. Even with the advent of the hydroforming process. the side rails continue to be manufactured from three or more components with substantially constant wall thickness.
It has been proposed to provide a polygonal shaped hydroformed component with the individual walls thereof having a distinct thickness in U.S. Pat. No. 5,557,961. However this patent requires pre-forming of a tubular member with an asymmetrical inner surface prior to the hydroforming process. With this process, the entire length of the component has the tailored thickness on each wall.